A switching power supply is widely used in an electronic device. For reducing the size and weight of the switching power supply, the frequency of the switching power supply is gradually increased. Consequently, the volumes of the passive components (e.g., inductors or capacitors) of the switching power supply are reduced. FIG. 1 is a plot illustrating the relationship between the driving loss and the switching frequency of a switching power supply. The driving loss of the switch element of the switching power supply is positively correlated with the switching frequency. As the switching frequency of the switching power supply is gradually increased, the driving loss gradually increases. In views of power-saving efficacy, the switching power supply of the electronic device should have enhanced efficiency even if the volume of the switching power supply is reduced. Consequently, it is important to reduce the driving loss of the switch element.
FIG. 2 is a schematic circuit diagram illustrating a driving circuit for a switch element according to the prior art. As shown in FIG. 2, the driving circuit comprises driving switches S11 and S12 and a resistor R. The capacitor Ciss is an input capacitor of a power switch (not shown). When the driving switch S11 is turned on, the input capacitor of the power switch is charged. When the driving switch S12 is turned on, the input capacitor of the power switch is discharged. The resistor R is a parasitic resistor of the charge/discharge loop.
FIG. 3 is a schematic circuit diagram illustrating a conventional metal oxide semiconductor field effect transistor (MOSFET). The voltage difference between the gate terminal G and the source terminal S of the MOSFET is Vgs. The voltage difference between the gate terminal G and the source terminal D of the MOSFET is Vgd. The capacitance of the input capacitor Ciss is expressed as: Ciss=Cgs+Cgd×(Vgd/Vgs). In the above formula, Cgs is the capacitance between the gate terminal and the source terminal S, and Cgd is the capacitance between the gate terminal G and the drain terminal D. Generally, the magnitude of the input capacitor Ciss corresponding to the voltage difference Vgs is provided from the specifications of the MOSFET.
The operations of the driving circuit will be described as follows. For charging the input capacitor Ciss, the driving switch S11 is turned on and the driving switch S12 is turned off. Consequently, the input capacitor Ciss is charged to a supply voltage Vcc through the driving switch S11 and the resistor R. The driving switch S11 is maintained in the on state and the driving switch S12 is maintained in the off state until the charging procedure of the input capacitor Ciss is completed. In the charging procedure, the energy consumed by the resistor R is equal to 0.5×Ciss×Vcc2. For discharging the input capacitor Ciss, the driving switch S11 is turned off and the driving switch S12 is turned on. Consequently, the input capacitor Ciss is discharged to 0V through the driving switch S12 and the resistor R. The driving switch S11 is maintained in the off state and the driving switch S12 is maintained in the on state until the discharging procedure of the input capacitor Ciss is completed. In the discharging procedure, the energy consumed by the resistor R is also equal to 0.5×Ciss×Vcc2.
For reducing the total consumed energy of the resistor R in the charging/discharging procedures, another driving circuit for the power switch is disclosed. FIG. 4 is a schematic circuit diagram illustrating another driving circuit for a switch element according to the prior art. FIG. 5 is schematic timing waveform diagram illustrating associated signals of the components of the driving circuit of FIG. 4. In this driving circuit, a constant current source is used for charging or discharging the input capacitor Ciss. For charging the input capacitor Ciss, the driving switches S21 and S23 are turned on in the time interval between t0 and t1. Consequently, the current flowing through the inductor L reaches a nearly-constant value I1. Then, the driving switches S21 and S23 are turned off. Consequently, in the time interval between t1 and t2, the inductor L provides the nearly-constant value I1 to charge the input voltage Ciss to the supply voltage Vcc. When the input voltage Ciss is charged to the supply voltage Vcc (i.e., at the time point t2), the driving switches S22 and S24 are turned on. Meanwhile, the charging procedure of the input voltage Ciss is completed. Since the current flowing through the resistor R is nearly constant in the charging procedure, the energy consumed by the resistor R is lower. For discharging the input capacitor Ciss, the driving switches S22 and S25 are turned on in the time interval between t2 and t3. Consequently, the current flowing through the inductor L reaches a nearly-constant value I2. Then, the driving switches S22 and S24 are turned off. Consequently, in the time interval between t3 and t4, the inductor L provides the nearly-constant value I2 to discharge the input voltage Ciss to 0V. When the input voltage Ciss is discharged to 0V (i.e., at the time point t4), the driving switches S21 and S23 are turned on. Meanwhile, the discharging procedure of the input voltage Ciss is completed. Since the current flowing through the resistor R is nearly constant in the discharging procedure, the energy consumed by the resistor R is lower.
However, since the inductor L has to provide the nearly-constant current in the charging procedure and the discharging procedure of the input capacitor Ciss of the power switch, lager inductance of the inductor L is required. That is, the inductor L has bulky volume. Moreover, since the current flows through the inductor L whenever the input capacitor Ciss is charged or discharged, the energy loss of the inductor L is larger.
Therefore, there is a need of providing an improved driving circuit for a power switch in order to overcome the above drawbacks.